A LEU(117)-]TRP MUTATION WITHIN THE RGD-PEPTIDE CROSS-LINKING REGION OF BETA-3 RESULTS IN GLANZMANN THROMBASTHENIA BY PREVENTING ALPHA-II-BETA-3 EXPORT TO THE PLATELET SURFACE

We report a case of Glanzmann thrombasthenia in a Pakistani child whos e platelets express less than 10% of the normal amount of alpha IIb be ta 3 on their surface. Single-stranded conformation polymorphism analy sis of the exons of the patient's alpha IIb and beta 3 genes showed an abnormality in exon 4 of the beta 3 gene, Direct sequence analysis sh owed that the patient was homozygous for a T --> G nucleotide substitu tion in this exon, resulting in the replacement of a highly conserved Leu at position 117 with Trp. Heterologous expression of alpha IIb bet a 3 containing the beta 3 mutation in COS-1 cells confirmed the pathog enicity of the Leu(117) --> Trp substitution and showed that it result ed in the intracellular retention of malfolded alpha IIb beta 3 hetero dimers. Additional site-directed mutagenesis at position 117 indicated that, although the smaller hydrophobic amino acid Vat could be substi tuted for the wild-type Leu, the larger hydrophobic amino acids Trp an d Phe or the charged amino acids Asp and Lys were not tolerated, These studies indicate that Leu(117) in beta 3 plays a critical role in att aining the correct folded conformation of alpha IIb beta 3. These stud ies also suggest that the hydrophobic side chain of Leu(117) is likely folded into the interior of beta 3, where it serves to stabilize inte rnal packing of the protein and determines its overall shape. (C) 1997 by The American Society of Hematology.